Where, exactly, is the line drawn?

[LupusDei]

However, you might also be able to establish the same dramatic tension in some other way. What if "sister" is "gf's" roommate, and she is just off limits because of that? What if "sister" is enrolled in a convent school, and "guy" thinks she is off limits because she is pursuing a religious vocation. The point is, you can probably compose a story that presents the same dramatic elements you are trying to present but doesn't necessarily involve the same biographical elements.

Probably. The guy is smitten but afraid. In his mind she screams trouble in all and any way. Ruling her underage and thus off limits is his attempt at escape from himself. Different reasoning with the same effect isn't impossible.

So from a storytelling point of view, it might be simpler to just say that they were born a year apart and were a year apart in school.

They are all three the same year at school. The gf is a year older than both others, but see it as important to hide that, including and especially from her bf, claiming to be a full year younger. Since she was forced to swith schools, sisters attend different schools.

Well, it probably worth a notice, the story as originally intended is set in ex-soviet territory, few years after the breakdown and re-won independence. It is still almost perfect anarchy, police is just the leading mob, laws have almost no meaning, and everyone is lying to everyone about anything, with or without any reason or purpose. Still the life goes on in surprising normalcy.